Re: A proposal to align GCC stack

["H.J. Lu" <hjl at lucon dot org>]

On Tue, Dec 18, 2007 at 06:31:25PM -0500, Ross Ridge wrote:
> Ye, Joey writes: 
> >i. STACK_BOUNDARY in bits, which is enforced by hardware, 32 for i386
> >and 64 for x86_64. It is the minimum stack boundary. It is fixed.
> 
> Ross Ridge wrote:
> >Strictly speaking by the above definition it would be 8 for i386.
> >The hardware doesn't force the stack to be 32-bit aligned, it just
> >performs poorly if it isn't.
> 
> Robert Dewar writes:
> >First, although for some types, the accesses may work, the optimizer
> >is allowed to assume that data is properly aligned, and could possibly
> >generate incorrect code ...
> 
> That's not enforced by hardware.
> 
> >Second, I am pretty sure there are SSE types that require
> >alignment at the hardware levell, even on the i386
> 
> This isn't a restriction on stack aligment.  It's a restriction on what
> kinds of machine types can be accessed on the stack.
> 
> As I mentioned later in my message STACK_BOUNDARY shouldn't be defined in
> terms of hardware, but in terms of the ABI.  While the i386 allows the
> stack pointer to bet set to any value, by convention the stack pointer
> is always kept 4-byte aligned at all times.  GCC should never generate
> code that that would violate this requirement, even in leaf-functions
> or transitorily during the prologue/epilogue.

>From gcc internal manual:

 -- Macro: STACK_BOUNDARY
     Define this macro to the minimum alignment enforced by hardware
     for the stack pointer on this machine.  The definition is a C
     expression for the desired alignment (measured in bits).  This
     value is used as a default if `PREFERRED_STACK_BOUNDARY' is not
     defined.  On most machines, this should be the same as
     `PARM_BOUNDARY'.

Since x86 always push/pop stack by decrementing/incrementing address
size, it makes senses to define STACK_BOUNDARY as address size. It
has nothing to do with application binary interface (ABI).

> 
> This is different than the proposed ABI_STACK_BOUNDARY macro which defines

The proposed ABI_STACK_BOUNDARY defines the value specified by the various
psABIs which gcc conforms.

> the possibily stricter aligment the ABI requires at function entry.  Since
> most i386 ABIs don't require a stricter alignment, that has ment that
> SSE types couldn't be located on the stack.  Currently you can get around
> this problem by changing the ABI using -fperferred-stack-boundary or by

No, gcc works around by setting

ix86_preferred_stack_boundary = 128;

by default.

> forcing an SSE compatible alignment using -mstackrealign or __attribute__
> ((force_align_arg_pointer)).  Joey Ye's proposal is another solution
> to this problem where GCC would automatically force an SSE compatible
> aligment when SSE types are used on the stack.
> 

Our proposal isn't just "another" solution. It is a solution for generic
stack alignment problems.


H.J.